It is now nearly 20 years since the Berlin Wall was breached, providing a powerful symbol of communism's impending collapse. That event also marked the end of an era in American defense planning, because the military challenge posed by the Soviet Union had taken most of the guesswork out of what kind of defense posture the nation needed.

When a hostile country has thousands of nuclear warheads aimed at you, it doesn't take much deliberation to realize you are in grave danger. So the main goal of U.S. strategy throughout the Cold War was to assure that those warheads would never reach America. Every facet of national-security activity, from counter-insurgency to conventional warfare to intelligence collection, was informed by the need to keep Soviet nukes in their silos.

Things are different today. There is no overwhelming threat around which to organize our defense preparations, despite the Bush Administration's attempts to make the global war on terror that crusade. History will record that once 9-11 focused the nation's attention on the challenge posed by "Islamo-fascism," Bush's team did a good job of keeping the terrorists at bay. But precisely because Al Qaeda has not managed to mount a follow-on attack in over six years, it cannot provide the central danger around which our defense preparations are organized.

Hence the administration's concept of capabilities-based planning, a tacit acknowledgement that we don't know which threats will be of greatest concern in the future.

In such circumstances, it makes sense for Congress to mandate a quadrennial review of defense programs and policies, to assure that military preparations remain in sync with changing security challenges. But we have now had four such exercises (including the 1993 Bottom-Up Review), and a pattern is beginning to emerge. The reviews that are conducted during the first year in office of a newly-elected President tend to be disorganized and incomplete. The reviews conducted during the fifth year of a Presidency that has recently seen its popular mandate renewed tend to be more deliberative and useful.

There are two obvious reasons for this disparity in results. First, it takes a long time to staff the security apparatus of new administrations, so conducting a comprehensive review during their first year in office means many key policymakers may not yet be in place. Administrations returning for a second term typically carry over many senior personnel.

Second, defense postures need to be driven by strategy, but the schedule mandated in law for delivering the findings of quadrennial reviews leaves little time to develop a strategy before the tradeoff of policies and programs must begin. Thus, there is a tendency to carry over the strategic assumptions of the preceding four years -- which doesn't work so well when a new President inherits unpopular policies.

That is the situation in which we find ourselves today, and it leads to an obvious conclusion: next year's Quadrennial Defense Review needs to be delayed until the new administration has time to staff policy positions and develop a new defense strategy. Rushing into another QDR a few months after Clinton or McCain or Obama is elected guarantees a sub-optimal result in which much of the work has to be done by career personnel using existing strategic concepts because new political appointees have not been confirmed and new thinking has not been formalized.

If Congress wants the QDR to achieve its desired result of tying military preparations closely to security challenges and requirements, then its timing needs to better reflect the way the political system actually works.